(a) Field of the Invention
The present invention relates to an activation circuit for a battery-operated security alarm detection system where current drain and, therefore, battery life is of concern. It employs a non-current drawing piezoelectric ("piezo") element which only sees part of the frequency band of interest to activate or wake up a current-drawing electret microphone which has a bandwidth broad enough to sees the entire audio band of interest. This circuit has particular importance for detectors which attempt to reduce false alarms by evaluating the signals present in a broad frequency band to look for specific expected multiple audio frequency signals which correspond to a specific event to be detected, such as a glass break.
(b) Description of the Prior Art
The electret microphone is often employed with audio frequency detectors which look at multiple frequencies to more accurately detect a specific event. This is because the electret microphone has a generally flat band-pass from about 20 Hertz ("Hz") to about 20 kiloHertz ("kHz"). However, the electret microphone requires a bias current of about 1 milliampere ("mA") to operate. Because of this constant current drain, known applications require hard-wiring the detectors containing an electret microphone to an external current source. There are no known applications of an electret microphone in a detector which employs battery operation, such as a 9 volt ("V") battery, because the battery life is very short. This invention solves this problem.